The invention relates to oscillator circuits useful in superheterodyne radio receivers. In particular the oscillator is useful in electronically tuned AM stereo receivers. In AM stereo the L+R, or sum information, is transmitted using conventional amplitude modulation. This signal is receivable on a conventional AM radio so that the stereo broadcast can be compatibly received with conventional or non-stereo receivers. The L-R, or difference information, is transmitted by phase modulation of the carrier. This signal information will be ignored in conventinal receivers. In a receiver equipped with a phase decoder the L-R signal can be recovered and matrixed with the L+R signal to reproduce L and R channel signals. Since the L-R channel is sent via phase modulation the stereo receiver must not introduce any appreciable phase jitter into the received signal. This places a severe requirement on the receiver local oscillator. Any local oscillator phase jitter will appear in the L-R signal as noise.
Another point is that for any high quality superheterodyne receiver a constant local oscillator injection level is desirable. In a typical radio broadcast receiver the local oscillator is tuned over a 2:1 range. That is, the tuning range is about twice the lowest frequency. The typical oscillator amplitude will vary by about 2:1 also. This makes constant local oscillator injection quite difficult.
Still another point relates to electronic tuned radio (ETR) circuits. Varactor tuning is commonly employed to make the circuits voltage tuned. Typically three varactors are gang tuned by a single control voltage. These are the local oscillator, the mixer input, and the r-f amplifier. These circuits can be made to track adequately so long as the signal voltages are small. Since the r-f and mixer circuits normally operate at a very low level they pose no problems. However, in the local oscillator the signal can be rather large. Such a large signal can act to shift the average tuning capacitance in a varactor and thereby upset the tracking. A small and controlled amplitude local oscillator signal would be very desirable.